Electron-beam devices, more particularly a scanning electron microscope (also referred to as an SEM in the following text) and/or a transmission electron microscope (also referred to as a TEM in the following text), are used to examine objects (specimens) for obtaining information with respect to the properties and the behavior in specific conditions.
In an SEM, an electron beam (also referred to as a primary electron beam in the following text) is generated using a beam generator and focused on an object to be examined by a beam-guiding system. A deflection apparatus is used to guide the primary electron beam in a raster over a surface of the object to be examined. In doing so, the electrons of the primary electron beam interact with the object to be examined. As a result of the interaction, in particular electrons are emitted by the object (so-called secondary electrons) and electrons in the primary electron beam are scattered back (so-called backscattered electrons). The secondary electrons and backscattered electrons are detected and used to produce an image. Thus, an image is obtained of the object to be examined.
In the case of a TEM, a primary electron beam is likewise generated using a beam generator and focused on an object to be examined using a beam-guiding system. The primary electron beam penetrates the object to be examined. As the primary electron beam passes through the object to be examined, the electrons in the primary electron beam interact with the material of the object to be examined. The electrons that passed through the object to be examined are imaged on a fluorescent screen or on a detector (e.g. a camera) using a system including an objective and a projection lens. Here, imaging can also occur in the scanning mode of a TEM. Such a TEM is generally referred to as a STEM. Additionally, in order to image an object to be examined, provision can be made for a further detector for detecting electrons that are scattered back from the object to be examined and/or secondary electrons that are emitted by the object to be examined.
Furthermore, the prior art has disclosed the use of combination devices, in which both electrons and ions can be guided onto an object to be examined, for examining objects. By way of example, it is known to equip an SEM with an ion-beam column as well. Ions are generated using an ion-beam generator arranged in the ion-beam column and are used for preparing an object (for example for ablating material from the object or for depositing material on the object) or else for imaging. The SEM here serves in particular for observing the preparation, but also for the further examination of the prepared or unprepared object.
There are specimens that, as a result of the material properties thereof, only allow imaging with a low contrast, when imaged using a particle-beam device or a light-optical device. By way of example, these specimens include organic specimens, more particularly biological material. In order to increase the contrast when imaging such a specimen, the prior art has disclosed the practice of staining such a specimen. By way of example, such a specimen is immersed into a diluted solution of osmium tetroxide or ruthenium tetroxide. The specimen remains in the solution for a certain amount of time. The specimen may remain in the solution for a duration of between several minutes and a number of hours. The specimen is subsequently taken out of the solution and washed in order to remove excess osmium tetroxide or ruthenium tetroxide, which has not combined with the specimen. However, the above-described procedure for staining the specimen is complicated.
As mentioned previously, combination devices are known that provide both an electron beam and an ion beam. In particular, the prior art has disclosed the use of a particle-beam device that is equipped with both an electron-beam column and an ion-beam column. The electron-beam column provides an electron beam, which is focused onto a specimen. Here the specimen is arranged in a specimen chamber, in which there is a vacuum. The ion-beam column provides an ion beam, which is likewise focused onto the specimen. The ion beam is used, for example, to remove a surface layer. Once the surface layer of the specimen has been removed, a further surface of the specimen is uncovered. A gas injection system is used to admit a precursor substance (precursor) of a stain (contrast agent), namely gaseous osmium tetroxide, into the specimen chamber. The further surface of the specimen is stained as a result of the interaction of the ion beam with the gaseous osmium tetroxide. In the process, a layer of osmium or an osmium-containing layer is substantially deposited on the further surface. The process of staining is completed by stopping the supply of the gaseous osmium tetroxide into the specimen chamber. Furthermore, osmium tetroxide remaining in the specimen chamber is pumped away. This is followed by imaging the further surface. The known method provides for a repetition of the above-described steps. Thus, a layer is once again removed from the surface until a further surface is uncovered in turn. This further surface is stained in turn and imaged using the electron beam.
However, the aforementioned prior art is disadvantageous in that the layer thickness of the contrast-agent layer respectively deposited on the surface can be embodied such that it may be difficult to examine structures of the specimen that are situated under a thick contrast-agent layer. Furthermore, the known methods always stain the whole surface of the specimen, particularly when the specimen is immersed. However, it is desirable to modify the contrast of only a very specific confined region of the surface of the specimen.
The prior art has also disclosed a measurement method for a semiconductor, in which provision is made for increasing a contrast in an image. In the known measurement method, material is removed from a semiconductor substrate using a focused ion beam in order to uncover a surface. When the material is removed using the focused ion beam, reaction products are created, which redeposit on the uncovered surface. However, this leads to a reduction in the contrast. In order to improve the contrast, the surface of the semiconductor substrate uncovered by the focused ion beam is simultaneously supplied with a fluorine-containing gas and a high-energy beam. This removes the reaction products that redeposit on the surface of the semiconductor substrate. This increases the contrast.
With respect to the prior art, reference is made to EP 1 890 136 A1, EP 1 890 137 A1 and U.S. Pat. No. 6,881,955 B2, which are incorporated herein by reference.
Accordingly, it would be desirable to specify a method for producing an image of a specimen, which allows an imaging of, where possible, all structures in a specimen with a sufficiently high contrast and which can be performed easily.